1. Field of the Invention
This invention relates generally to medical ventilators and control systems for medical ventilators, and more particularly concerns a system and method for adaptive inverse control of pressure based ventilation.
2. Description of Related Art
A patient receiving breath pressure support from a ventilator system typically receives breathing gas through a patient circuit of the ventilator. The patient circuit generally consists of two flexible conduits connected to a fitting called a patient wye. The free ends of the conduits are attached to the ventilator so that one conduit receives breathing gas from the ventilator""s pneumatic system, and the other conduit returns gas exhaled by the patient to the ventilator. The volume of the exhaled gas may then be measured in a spirometer before it finally exits through an exhalation valve. The wye fitting is typically connected to the patient""s breathing attachment or enclosure, which conducts breathing gas into the lungs, and conducts exhaled gas from the lungs to the exhalation branch of the patient circuit. The pneumatic system at the inspiratory end of the patient circuit is typically closed before a breath, and the exhalation valve at the exhalation end of the patient circuit is typically preceded by a one-way valve, to prevent gas from flowing retrograde in the exhalation branch of the patient circuit.
In pressure based ventilation, occurrences of low pressures in the exhalation limb of the patient""s breathing gas circuit during the exhalation phase of the pressure supported breath can be a cause of concern for the patient unless they are carefully controlled. Pressures in the patient lung that fall below PEEP (Positive End Expiratory Pressure, a baseline pressure value) can impair a patient""s lung function, and it can be important to maintain PEEP in a patient""s lung to prevent collapse of the lung.
On the other hand, another current problem encountered in pressure based ventilation (PBV) is the control of overshoot of patient airway pressure during the inspiration cycle for pressure controlled type of ventilation. When the breath cycle is primarily controlled by the standard pressure and flow controllers, the patient can experience an overshoot in the airway pressure as a controller commands the flow valve opening to rapidly supply breathing gas to the patient during the initial phases of the inspiration. Thus, it is highly desirable to provide more accurate control of patient airway pressure during pressure based ventilation, since pressures that are too high or too low can have a number of undesirable effects.
Various types of adaptive controllers are known that have been used for controlling the functions of a patient ventilator. One type of adaptive control system for a medical ventilator for controlling flow and pressure based on the dynamic state of the ventilator utilizes a waveform generator that provides a desired pressure waveform. The flow control system has a switching logic block which can adaptively select either a flow delivery control algorithm or a flow exhaust control algorithm, based on the ratio of the pressure to the flow of gas. Both the delivery and exhaust control systems comprise feedback loops for minimizing the error between a target pressure signal from the waveform generator and a feedback signal corresponding to actual airway pressure.
In another type of adaptive controller for automatic ventilators, a pulse oximeter is used to optically determine hemoglobin saturation of the patient""s blood, and the information is used to regulate the length of inspiration time, PEEP, and the fraction of inspired oxygen (FiO2) supplied to a patient""s breathing tube.
Another type of adaptive controller utilizes a feedback control loop having a transfer characteristic that is adaptive to the changing requirements of the patient so as to maintain the proper fractional amount of oxygen inspired. A closed loop non-invasive oxygen saturation control system is provided that utilizes an oximeter, and an algorithm that provides an adaptive filtering of the oxygen values for feedback control.
One known adaptive feedback control for a medical ventilator analyzes in the frequency domain the Laplace transfer functions that are determined for the elements of the ventilator system. To accurately control the mouth pressure, adaptive feedback control is used so that the control""s lag term cancels the patient""s lead term effect on the system. To precisely control the back pressure applied to a diaphragm valve, an expiratory pneumatic circle and control loop use a second closed loop controller. A control valve is operated based upon a pressure error signal generated by the controller. The inspiratory control function uses a control function algorithm, synchronization algorithm, time constant algorithm, and a resistance/compliance algorithm.
One of the most enduring and difficult problems in pressure based ventilation has been achieving pressure tracking performance across a wide range of patient conditions and ventilator settings. Non-stationary patient parameters, ventilator settings, and a wide selection of connecting circuit types can cause system dynamics to vary by several orders of magnitude. Classical control methods which use fixed gain controllers are typically unable to maintain consistent performance as these parameters vary. The designer must often resort to ad hoc modifications which, while marginally improving robustness, will likely require tradeoffs between overshoot of the trajectory, rise time, steady state accuracy, and stability margin. There thus remains a need for improving the control of pressure based ventilation.
The present invention meets these and other needs.
Briefly, and in general terms, the present invention provides for an adaptive control method which solves the pressure based ventilation control problem. This method utilizes a specific regulator structure designed according to a linear model of the patient/ventilator system. The same model is also used to derive a real time estimate of patient airway resistance, lung compliance, and the connecting tube compliance. These estimates are used to adjust regulator parameters in real time such that closed loop dynamics quickly approach those of a specified system. In the patient/connecting circuit system, inlet flow and circuit pressure are utilized for estimation of parameters for adjusting the pressure of gas to the desired pressure. The present invention provides for a unique gas pressure compensation, and a unique parameter estimation for providing parameters for compensation of the pressure of gas to the desired pressure, to provide for improved pressure based ventilator control. The present invention is able to provide consistent performance in pressure tracking over the entire range of patient conditions from small infant to large adult without having to balance the tradeoffs mentioned above. As a side benefit, the invention also provides patient and connecting circuit parameter measurements which can be used at a higher level in the ventilator control and can be used to possibly eliminate any need to run a system self test (SST) to determine these parameters.
While the method and apparatus of the present invention is described in the context of control of the inhalation cycle of a pressure based ventilation system, the method may also be applied to the exhalation cycle of both pressure based and volume controlled ventilation strategies. By modeling the exhalation aspects of the patient ventilator system using the methods of the invention, regulator parameters may be adjusted to quickly obtain a desired exhalation trajectory.
These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings, which illustrate by way of example the features of the invention.